The understanding of neuronal cell differentiation and its regulation remains a central problem in developmental neurobiology. Advances in molecular genetic technology have allowed investigators to develop systems for examing cell differentiation and its regulation in non-neural tissues. Application of these techniques to study the mammalian nervous system requires a sufficient population of single-type neuroblasts that will synchronously differentiate into fully functioning neurons. Adult rat olfactory neuroepithelium provides such a system, since the capacity for forming functional sensory neurons from stem cells persists into adult life. The long term goal is to develop this system to permit definition and eventual characterization of genes that play important roles in development of the olfactory neuroepithelium. These studies will also lead to a more general understanding of neuronal differentiation and development. The basal cells of the olfactory neuroepithelium, identified as the stem elements of the sensory neurons, continually divide and differentiate to fully functioning neurons. Among nerve cells of adult mammals, the olfactory elements are unique in their capacity to turn over and be replaced after injury. Analysis of these proteins and their genes may reveal fundamental differences between the olfactory system and other neuronal systems. In addition, this process appears to recapitulate the olfactory embryogenic development which may provide insight into more general questions of neuronal development. This proposal seeks specifically 1) to utilize immunocytochemical and in situ hybridization techniques to characterize the mature neuroepithelium with available neuronal specific markers. 2) to use the neuronal specific hybrid-toxin (tetanus toxin corss-linked to the ricin A chain, a potent eucaryotic protein inhibitor) for olfactory receptor cell killing and to characterize both morphologically and biochemically the subsequent differentiation and maturation process. Application of the results from the above aims should allow: 3) determination of the expression and regulation of neuronal specific markers during differentiation. 4) identification by differential cDNA screening of specific mRNA's that are expressed during the differentiation process. It is expected that the proposed studies will lead to the development of a well-characterized mammalian neuronal differentiation system in which to apply molecular genetic technology. Such a system will provide further insight into the understanding of the development of the normal mammalian nervous system.